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Show MARVINE ON SUPERIMPOSED VALLEYS. 165 structure and characters of the upper rocks, and when they gradually cut down through these and commenced sinking their canons into the underlying complicated rocks, these canons bore no relation whatever to their complications. It is but recently that the upper rocks have been completely removed from the summits of the mountain-spurs, the ancient level of subaqueous erosion being still indicated by the often uniform level of the spurs and hill-tops over considerable areas, and large plateau-like regions which became very marked from certain points of view. Two or three such levels are indicated at a few places, showing not only that the sedimentaries have once extended up over what are now the mountain rocks, but that the uplifting has been mainly confined along certain partly well-defined lines, the intermediate belts, though uplifted bodily, remaining comparatively level, a type of folding, probably, not uncommon farther west, and which will be referred to again in the following chapter. * •,."•*. ...•*• •''.*• * #.'••-.••. * "It is true that the structure of the lower rocks has begun to affect the courses of the streams, and in places to a considerable extent. Meeting a softer bed a canon will often have its course directed by it, and follow it for some distance, leaving the adjacent harder beds plainly indicated by the ridges, and sometimes the sinuosities of structure are very curiously followed by a stream in all' its windings, but it soon breaks away and runs independently of the bedding. Many of the smaller ravines have had their positions determined by the structure; but in a broad sense the drainage is from the main mountain crest eastward, independent of structure. Thus, while in places geological features may find expression in surface form, yet, as often, there may be no conceivable relation between topography and geology. The subaqueous erosion, in smoothing all to a common level, destroys all former surface expression of geological character, and the present erosion has not yet been in progress sufficiently long to recreate the lost features/' I fully concur with Mr. Marvine in the above explanation of the valleys in the main Rocky Mountains of Colorado, as my own observations in that country had led me to the same conclusion. There can be no doubt that the present courses of the streams were determined by conditions not found |
